The present invention relates to a lead acid battery.
As a collector for positive plate to be incorporated in a lead acid battery there has heretofore been used a Pbxe2x80x94Sb alloy. However, a battery using such an alloy is subject to drastic self-discharging or electrolyte loss and thus can hardly be used in the form of maintenance-free or valve-regulated battery. In order to inhibit self-discharging or electrolyte loss and hence render the battery maintenance-free, a Sb-free lead alloy such as Pbxe2x80x94Ca alloy comprising Ca in an amount of from 0.04 to 0.12 wt % has been used.
However, a battery comprising a positive collector made of a Sb-free lead alloy can be subject to capacity loss earlier than a battery comprising a Pbxe2x80x94Sb alloy. This is presumably attributed to poor adhesion between positive collector and active material, corrosion layer formed on collector which has high reactivity, and so on. Anyway, the battery comprising a Sb-free positive collector is disadvantageous in that the corrosion layer formed on the collector can easily discharge to form a passivated layer on the interface of the collector with the active material, making it impossible for the active material to sufficiently discharge.
Accordingly, in order to improve the life of the battery comprising a positive collector made of a Sb-free lead alloy, it is considered effective to improve the adhesion between the positive collector and the active material or form a corrosion layer which can hardly cause electric discharge. As one of approaches for improvement, it is proposed that a Pb-Sc alloy layer be formed on the on surface of a Pbxe2x80x94Ca alloy positive collector (JP-A-63-148557 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)). It is known that the use of this approach makes it possible to prevent premature capacity loss and improve the resistance to electrolyte loss as compared with the battery comprising a collector made of a Pbxe2x80x94Sb alloy. However, even this approach is disadvantageous in that the collector used contains Sb, though only in the surface layer of the collector, which is dissolved with the electrolyte and deposited on the negative electrode to lower the hydrogen overvoltage, making it impossible to inhibit self-discharging or electrolyte loss as in Pbxe2x80x94Ca allay collector. In particular, the use of this approach in the use where charging is effected at a constant voltage is disadvantageous in that thermal runaway or drying-up (dry out) of electrolyte occurs to shorten the life of the battery.
Accordingly, the positive collector is required to exhibit (1) desired mechanical strength and workability, (2) corrosion resistance, (3) adhesion to active material, (4) resistance to passivation on the interface of collector with active material, (5) resistance to drop of hydrogen overvoltage on the negative electrode attributed to the positive electrode, and so on, on condition that it has a proper electrical conductivity.
An object of the invention is to provide a lead acid battery which satisfies the foregoing requirements as a whole of the positive collector and exhibits a prolonged life.
The foregoing object of the invention is accomplished with a collector for lead acid battery of the first embodiment of the present invention comprising a substrate and a surface layer which has an alloy composition different from that of the substrate and which is formed on at least a part of the surface of the substrate, characterized in that the substrate is a Sb-free lead alloy or lead and the surface layer is a lead alloy layer comprising one or more metals selected from the group consisting of alkaline metals and alkaline earth metals.
In the arrangement of the foregoing positive collector, the substrate mainly satisfies the desired mechanical strength, workability and corrosion resistance of the whole of the collector and the surface layer mainly improves the adhesion between the collector and the active material and resistance to passivation on the interface of the collector with the active material. Further, neither the substrate nor the surface layer contains Sb. Accordingly, the requirements for prevention of drop of hydrogen overvoltage on the negative electrode attributed to the positive electrode can be satisfied.
In the second embodiment, the positive collector for lead acid battery of the first embodiment comprises Ca in the surface layer in a content of from 0.2 to 5 wt %.
In the third embodiment, the positive collector for lead acid battery of the first embodiment comprises Mg in the surface layer in a content of from 0.1 to 5 wt %.
In the fourth embodiment, the positive collector for lead acid battery of the first embodiment comprises K in the surface layer in a content of from 0.1 to 5 wt %.
In the fifth embodiment, the positive collector for lead acid battery of the above embodiments comprises the surface layer further comprising a lead alloy comprising Sn in an amount of from 0.1 to 30 wt %.
In the sixth embodiment, the positive collector for lead acid battery of the above embodiments comprises the substrate comprising a lead alloy or lead metal comprising Ca in an amount of from 0 to 0.12 wt % and/or Sn in an amount of from 0 to 3 wt %.
In the seventh embodiment, the positive collector for lead acid battery of the above embodiments has the thickness of the surface layer of 1 xcexcm or more and 30% or less of that of the substrate.
In the eighth embodiment, the positive collector for lead acid battery of the above embodiments comprises the surface layer which is formed on the surface of the positive collector to one sixth of surface area of the positive collector.
In the ninth embodiment, the positive collector for lead acid battery of the above embodiments comprises the collecting tab of the collector which is free of the surface layer at least on the point at which it is welded and integrated to a strap.
In the tenth embodiment, the positive collector for lead acid battery of the above embodiments comprises the positive collector which is a sheet-like lead alloy or lead or a grid obtained by expanding or punching a lead alloy or lead sheet.
In the eleventh embodiment, the positive collector for lead acid battery of the tenth embodiment is obtained by working an integrated lead alloy sheet prepared by rolling a laminate of lead alloy sheets having one or more metals selected from the group consisting of alkaline metals and alkaline earth metals on one or both surfaces of Sb-free lead alloy or lead substrate.
In the twelfth embodiment, the lead acid battery comprises a positive collector for lead acid battery of the above embodiments.
In the thirteenth embodiment, the lead acid battery of the twelfth embodiment comprises an active material comprising at least one alkaline metal or alkaline earth metal compound incorporated therein.
In the fourteenth embodiment, the lead acid battery of the fifth embodiment comprises an active material comprising an Sn compound incorporated therein.
In the fifteenth embodiment, in the lead acid battery of the thirteenth embodiment, as the starting material of active material a lead powder prepared from a lead alloy comprising alkaline metals or alkaline earth metals is used.
In the sixteenth embodiment, in the lead acid battery of the fourteenth embodiment, a lead powder prepared from a lead alloy comprising Sn is used as the starting material of active material.
In the seventeenth embodiment, in the lead acid battery of the twelfth embodiment, an electrolyte contains alkaline metal or alkaline earth metal ions.
In the eighteenth embodiment, the lead acid battery of the fifth embodiment comprises an electrolyte containing Sn ions.